1. Field of the Invention
The present invention relates to apparatus (devices) for controlling and guiding waves such as waves of electromagnetic radiation, acoustic waves, pressure waves and the like. This apparatus includes at least one wave-transmitting medium and a diffraction grating, associated with the transmitting medium, for scattering wave energy into or out of the guided waves. The apparatus may take the form a "sandwich"-type or thin film waveguide, in which case the transmitting medium has at least one curvi-planar boundary. The apparatus may also take the form a "rod"-shaped or cylindrical waveguide, in which case the transmitting medium has a substantially cylindrical boundary.
2. Description of the Prior Art
Wavelength and direction-selective diffraction of acoustic or electromagnetic waves is conveniently achieved by an arrangement called a "diffraction grating" or "Bragg reflector". A "diffraction grating" is comprised of a large number of substantially parallel and equidistant "grating lines" which produce controlled scattering of waves. The "grating lines" may be created by variations in relief of a wave-reflecting surface, or by variations in some aspect of a wave-transmitting medium which varies some aspect of the waves propagating within the medium. For example, the lines may be formed by variations in the index of refraction--which determines the velocity of waves within the transmitting medium--or variations in the "gain constant"--i.e., the constant of wave amplification or attenuation--at successive lines or points within the medium.
A one-dimensional grating consists of a plurality of parallel, equidistant and linear grating lines on a transparent or reflecting planar or concave surface. The grating lines may be alternatively more or less absorbing, giving an amplitude grating, or varying in relief or refractive index in an alternating manner, giving a phase grating. A two-dimensional grating may be formed, for example, by a pair of one-dimensional gratings superimposed at a certain angle or by circular or spiral grating lines provided on a planar surface. Three-dimensional diffraction gratings occur naturally, for example in crystals, due to the ordering of molecules along straight lines.
Joseph Fraunhofer is generally regarded as the inventor of diffraction gratings. He described small grating lines on a planar, reflecting surface in 1823, developed their geometrical theory and used them to make surprisingly accurate wavelength calibrations of solar absorption lines.
Today, diffraction gratings are used in a large variety of devices, particularly optical devices. Typical of this large body of prior art are the following references:
(1) U.S. Pat. No. 3,970,959 to Wang et al. entitled "Two Dimensional Distributed Feedback Devices and Lasers", 1976.
(2) U.S. Pat. No. 4,006,432 to Streifer et al. entitled "Integrated Grating Output Coupler in Diode Lasers", 1977.
(3) U.S. Pat. No. 4,140,362 to P. K. Tien entitled "Forming Focusing Diffraction Gratings for Integrated Optics", 1976.
(b 4) J. Dyson, "Circular and Spiral Gratings" Proc. Royal Soc. London Ser. A248, pp. 93-106, 1958.
(5) C. V. Shank, and R. V. Schmidt, "Double-Heterostructure GaAs Distributed-Feedback Laser", Appl. Phys. Lett. 25, pp. 200-201, 1974.
(6) H. A. Haus, and C. V. Shank, "Antisymmetric Taper of Distributed Feedback Lasers", IEEE J. Quantum Electron. QE 12, p. 532, 1976.
(7) R. F. Kazarinov et al., "Planar Distributed-Feedback Optical Resonators", Sov. Phys. Tech. Phys. 21, pp. 130-136, 1976.
The U.S. Pat. No. 3,970,959 to Wang et al. teaches the application of a two-dimensional diffraction grating to thin film optical wave guides. These gratings serve to reflect or scatter light waves into controllable transverse modes of propagation. The U.S. Pat. No. 4,006,432 to Streifer et al. utilizes a one-dimensional grating in a heterojunction diode laser to produce a highly collimated, polarized light beam perpendicular to the plane of the PN junction (which is the plane of the thin film waveguide structure). The U.S. Pat. No. 4,140,362 to Tien discloses techniques for producing two-dimensional curved line diffraction gratings on a thin film optical device. The purpose of such gratings is to focus as well as diffract light that is confined to the film--i.e., the "optical waveguide"--in which the grating is formed. Although these grating lines produced by Tien form only a segment of a circle, completely circular and spiral diffraction gratings are also known from the article by Dyson (Ref. 4, above).
Resonators for optical wave lasers have been built by replacing the end mirrors by diffraction gratings, eventually combining them into one grating extending over the entire length of the light-amplifying medium (Ref. 5, above). By properly adjusting the shift of two gratings with respect to each other, single mode operation at one of the Bragg wavelengths of the grating can be achieved (Ref. 6, above). Furthermore, a planar film waveguide containing a one-dimensional grating has been considered as an optical resonator with laser emission at an angle to the plane of the film waveguide (Ref. 7, above).
It is not known, however, to device a diffraction grating within a thin film (curvi-planar) or rod-shaped resonator so as to efficiently couple power out of the resonator. More particularly, it is not known to couple power out of a curvi-planar resonator into an optionally linearly polarized and optionally focussed beam directed substantially vertical to the plane of the resonator. With rod-shaped structures, it is not known to produce standing waves within these structures, thereby to produce a resonator, by means of gratings. It is also not known to effect the efficient coupling of power out of such a resonator with an omni-directional beam having maxima substantially perpendicular to the axis of the resonator.
The application of nonlinear optics to optical waveguides is of great current interest, since the waveguide confines the optical intensity to a small area given by the cross-section of the waveguide and maintains a high optical intensity over a longer propagation distance. It is not known, however, to further concentrate the optical intensity within a waveguide by a diffraction grating. More particularly, it is not known to concentrate in a thin film waveguide optical intensity in the center of a standing wave pattern produced by a cylindrical diffration grating.